The objective of the proposed research is an elucidation of how alterations in various peripheral and central neural and humoral factors interact in generating breath-by-breath modulations of airway smooth muscle tone. The breath-by-breath changes in airway size are measured from pressure changes within pliable cuffs mounted on tubes and catheters advanced in various airways. The influence of information from pulmonary stretch receptors (PSE) is of especial interest. In anesthetized, paralysed dogs the discharge patterns of PSR are altered by varying the rate of change of a given volume as well as the ratio of inspiratory time to expiratory time. Changes in the pressure cuffs will be compared to simultaneous measurements of airway resistance to ascertain the relationship between the two parameters. Cuff pressures recorded during systemic manipulations will be used in combination with in vivo measurements of mechanical and electrical events of trachealis muscle. The use of these several indices of airway smooth muscle tone (cuff pressure, mechanical and electrical activity), will permit a better understanding of the mechanisms involved in regulation of distribution of an inspirate within different lung regions. We postulate that this control mechanism is contributory to minimize dispersion of ventilation-perfusion ratios in the lung.